There is a bug where, when loading a saved game or moving to a completely different zone (such as another map), some units do not appear. They exist—you can see their shadows—but they are literally just moving shadows. You cannot select them in any way. If they are enemy units, they will attack you and can take damage from area-of-effect spells or basic attacks, but they cannot be selected in any way, nor can they be targeted by single-target spells.
This issue has been happening very frequently in custom campaigns, and in many different ones. At first, I thought it was a bug specific to a certain campaign, but it happens in every single one of them—even in those where it didn’t occur before, despite no updates in between. This only happens when loading a saved game or when transitioning to another map through a transfer zone (such as the ones in Rexxar’s campaign).